1. Field of the Invention
This invention relates broadly to improved force measuring apparatus. More particularly, this invention relates to apparatus for measuring horizontal- and vertical-roll forces in a universal rolling mill stand.
2. Description of the Prior Art
It is important to operators of both old and new metal rolling mills to know the magnitude of roll forces, and other parameters, that will be present in a mill stand after adjusting work roll gap to ensure a desired reduction of a workpiece during passage through the mill stand. In older mills, mechanical roll gap indicator readings, together with other parameter information, are the only way of ultimately determining the value of roll forces for a given set of operating conditions. Frequently, operating conditions change and produce substantial errors in the roll force so determined. In some newer strip mills for example, simple load cells are incorporated into the mill stand structure at great expense and are connected to remote reading roll force indicator. This indicator enables the operator to read roll forces to certain accuracies, and with constant attention, will enable the operator to prevent overloading of work rolls and mill stand structures.
In universal rolling mills where structural shapes, such as "I" and "H" beams are rolled, the determination of roll forces is a complex matter. This is because mill stands have both horizontally- and vertically-aligned work rolls which produce two kinds of roll forces in mill side frames simultaneously during rolling operations. Hereinafter these two kinds of roll forces will be referred to as horizontal-roll forces F.sub.H and vertical-roll forces F.sub.V and are defined as follows. Horizontal-roll forces F.sub.H act vertically in a mill side frame and are exerted on the workpiece through a pair of horizontally-aligned work rolls by means of a screwdown mechanism which adjusts the vertical gap between these rolls to control one dimension of the structural shape. Vertical-roll forces F.sub.V act horizontally in a mill side frame and are exerted on the workpiece through a pair of vertically-aligned work rolls by a separate screwdown mechanism which adjusts the horizontal gap between these rolls to independently control a second dimensional reduction of the structural shape. Symmetrically shaped workpieces produce substantially equal F.sub.H or F.sub.V roll forces in a pair of end posts of opposing mill side frames. Occasionally, the F.sub.H or F.sub.V roll forces may be unequal in the same pair of end posts due to a special shape being rolled.
Generally, in a universal rolling mill, horizontal-roll forces F.sub.H produce tension stresses in a given pair of mill end posts of opposing mill side frames, while vertical-roll forces F.sub.V produce bending stresses in the same pair of end posts. Whenever roll forces are either equal or unequal, so are corresponding stresses. This simultaneous combination of F.sub.H and F.sub.V roll force stresses produces complex stress patterns in the end posts which are difficult to measure. This measuring difficulty is further compounded by a zero drift component generated by thermal and mechanical operating variations such as mill stand warpage and mill stand hysterisis. Operating variations causing the zero drift component may occur prior to, during and/or between rolling schedules.
It has been discovered that the operating variations causing the zero drift component affect stress patterns at various strain sites on individual mill posts in different ways. At opposite strain sites a given end post the zero drift component at each site may be opposite each other and may drift into an equal or even a reverse condition. Strain sites on another end post may respond differently than the first post at any given time. After sufficient rolling time, the zero drift component may even stabilize at a different value for each strain site on each end post. Consequently, when using strain gages at the end post strain sites in a universal rolling mill, the variable zero drift component also affects the horizontal- and vertical-roll force F.sub.H and F.sub.V zero references in a corresponding way.
Prior art apparatus has limited provisions for measuring only horizontal-roll forces in a rolling mill having only horizontal work rolls. For example, either an electromagnetic or a strain gage type of extensometer was attached to only one surface of a mill post to sense the horizontal-roll force therein, it being heretofore assumed that the horizontal-roll forces in the other post were the same as the first post. The extensometer output signal was connected to a zero drift corrector, using either electromagnetic or electronic means, before being indicated or recorded as roll force.
In a more recent example of prior art, four half-bridge strain gages mounted off-center on opposite sides of two mill posts in a horizontal-only rolling mill. These gages are connected into two full-bridge circuits so that they cancel the effects of bending stresses in the two mill posts. Their output signals represent only horizontal-roll forces in the two mill posts and are electronically corrected for a zero drift component on a per-post basis. Horizontal-roll force signals are selectively connected to means for indicating either the sum of, or individual, mill post roll forces.
Neither of the foregoing prior art arrangements work satisfactorily in a universal rolling mill environment because they fail to provide operating needs in a contemporary mill which may have either old or new rolling mill equipment. That is, they do not provide simultaneous indications of both horizontal- and vertical-roll forces, individual force sensor zero drift correction, nor means for resolving complex stress patterns into relative simple roll force components that will satisfy the needs of universal rolling mill operators.